Читать книгу Poly(lactic acid) - Группа авторов - Страница 135

As predicted from the phase diagram (Figure 6.1), quenching the melt gives various crystalline phases depending on the quenching temperature. The X‐ray diffraction measurement revealed the details as shown elsewhere [5]. The mesophase was formed when the quenching temperature was near T _g. By quenching into 100–120°C, the δ crystal was formed. Cooling to the higher temperature caused crystallization to the α form.

Figure 6.12a and b show the time dependence of the infrared absorbance at 921 cm⁻¹ band, which is common to various crystal phases, measured in the temperature jump process from the amorphous phase or from the melt [5]. In these experiments, the temperature was changed quite sharply to a preset crystallization point by using a homemade temperature jump cell [56]. The crystallization rate k was estimated from the steepest slope of the intensity‐vs‐time curves at each temperature, as seen in Figure 6.12c. The crystallization rate started to increase above 70°C and showed a maximum at around 110°C, and then decreased with increasing temperature. These curves are similar to those of the spherulite growth rate measured with an optical microscope [50, 57]. The two important points are extracted from these experimental data. One is about the crystallization of the different crystal forms: in the low temperature region (70–110°C), the δ form is produced, while the α form is crystallized in the high temperature region (>110°C), as already mentioned above. Another point is about the crystallization rate. As seen in Figure 6.12c, the cold‐crystallization gave the crystallization rate higher than the melt‐crystallization when compared at the same crystallization temperature. The higher crystallization rate in the cold crystallization phenomenon was ascribed to the higher content of the crystalline nuclei produced in the cooling process from the amorphous phase compared with that from the molten state [58, 59].

FIGURE 6.10 Temperature dependence of 2D X‐ray diffraction patterns of the oriented PLLA mesophase measured in the heating process.

Source: Reproduced from Wasanasuk et al., Macromolecules 2011, 44, 9650–9660.

FIGURE 6.11 The temperature dependence of the crystallite size estimated for the equatorial and meridional directions of the oriented PLLA sample during the cold crystallization process starting from the meso form.

Source: Reproduced from Wasanasuk et al., Macromolecules 2011, 44, 9650–9660.